2.7 DNA Autoreproduction
Prior to division, the cell’s DNA is replicated in its entirety and splits apart,
resulting in the formation of two identical daughter cells (the name given to the
cells produced through cell division). If the same DNA is not replicated, the result
will be different kinds of cells. In 1940, Linus Pauling and M. Delbrück speculated
that the surface of the genetic material provided a template, creating molecules of
complementary shape that then became their own template, resulting in the creation
of a material identical to the original (Pauling and Delbrück 1940). It was against
this backdrop that Watson and Crick presented their double helix model for DNA in
1953, after which speculation as to DNA’s replication mechanism was based on that
model.
Because DNA consists of genes, it is located in the cell nucleus and is a component of the chromosome. When the cell divides, chromosomes are split equally
and distributed to two cells, both of which must have the same DNA both in
quantitative and qualitative terms. To ensure this, DNA must be precisely replicated
prior to division to produce two identical molecules. This is known as DNA
autoreproduction, where the double helix serves a structure to allow this process to
occur (Fig. 2.11).
When autoreproduction begins, the hydrogen bonds between bases are first
broken apart by helicase and two strands of main chains separate. In the gap
between them gather the four nucleotides that make up DNA. These form complementary pairs with the bases from each separated chain, which are linked
through hydrogen bonds. The sugar components of each nucleotide are subsequently linked with phosphate and a long chain is formed. Obviously, several
enzymes, including those related to DNA synthesis, are involved in this synthesis
process.
As a result of this, a new, complementary polynucleotide chain is synthesized
along the entire length of each original DNA strand, resulting ultimately in the
formation of two DNA molecules with the exact same base pair arrangement. In
other words, the two existing DNA strands function as templatess for synthesis of a
new chain. Because each of the two newly synthesized DNA molecules contains a
strand from the original DNA chain that served as a template, this is referred to as
“semiconservative replication.” Watson and Cricks’ double helix structure was thus
capable of explaining the various properties and functions observed for DNA to
date, and it was immediately recognized as the prevailing theory.
E. coli DNA consists of around 4.2 million nucleotide pairs, measuring
approximately 1.4 mm in length and consisting of a ring with its two ends connected (Galau et al. 1974). These are packed tightly within the cell, which measures
around 0.0007 mm across and 0.001–0.004 mm in length. At the same time, it has
no cell nucleus surrounding it. This form of organism possessing DNA but no
nuclear membrane is known as a prokaryote; examples include bacteria and blue–
green algae. In other organisms, proteins known as histones combine with DNA
molecules. These are divided into set numbers; when division occurs, they cluster to
2.7 DNA Autoreproduction
37
Prior to division, the cell’s DNA is replicated in its entirety and splits apart,
resulting in the formation of two identical daughter cells (the name given to the
cells produced through cell division). If the same DNA is not replicated, the result
will be different kinds of cells. In 1940, Linus Pauling and M. Delbrück speculated
that the surface of the genetic material provided a template, creating molecules of
complementary shape that then became their own template, resulting in the creation
of a material identical to the original (Pauling and Delbrück 1940). It was against
this backdrop that Watson and Crick presented their double helix model for DNA in
1953, after which speculation as to DNA’s replication mechanism was based on that
model.
Because DNA consists of genes, it is located in the cell nucleus and is a component of the chromosome. When the cell divides, chromosomes are split equally
and distributed to two cells, both of which must have the same DNA both in
quantitative and qualitative terms. To ensure this, DNA must be precisely replicated
prior to division to produce two identical molecules. This is known as DNA
autoreproduction, where the double helix serves a structure to allow this process to
occur (Fig. 2.11).
When autoreproduction begins, the hydrogen bonds between bases are first
broken apart by helicase and two strands of main chains separate. In the gap
between them gather the four nucleotides that make up DNA. These form complementary pairs with the bases from each separated chain, which are linked
through hydrogen bonds. The sugar components of each nucleotide are subsequently linked with phosphate and a long chain is formed. Obviously, several
enzymes, including those related to DNA synthesis, are involved in this synthesis
process.
As a result of this, a new, complementary polynucleotide chain is synthesized
along the entire length of each original DNA strand, resulting ultimately in the
formation of two DNA molecules with the exact same base pair arrangement. In
other words, the two existing DNA strands function as templatess for synthesis of a
new chain. Because each of the two newly synthesized DNA molecules contains a
strand from the original DNA chain that served as a template, this is referred to as
“semiconservative replication.” Watson and Cricks’ double helix structure was thus
capable of explaining the various properties and functions observed for DNA to
date, and it was immediately recognized as the prevailing theory.
E. coli DNA consists of around 4.2 million nucleotide pairs, measuring
approximately 1.4 mm in length and consisting of a ring with its two ends connected (Galau et al. 1974). These are packed tightly within the cell, which measures
around 0.0007 mm across and 0.001–0.004 mm in length. At the same time, it has
no cell nucleus surrounding it. This form of organism possessing DNA but no
nuclear membrane is known as a prokaryote; examples include bacteria and blue–
green algae. In other organisms, proteins known as histones combine with DNA
molecules. These are divided into set numbers; when division occurs, they cluster to
2.7 DNA Autoreproduction
37